1. Field of the Invention
The present invention pertains to implantable medical devices, and more particularly to a data transfer and storage apparatus for use with an implanted cardiac device such as a defibrillator or pacemaker.
2. Description of the Prior Art
The cardiac device data acquisition apparatus, commonly referred to as an interrogator, is used by a physician, typically a cardiologist or surgeon, to monitor the program setting of an implanted device and/or to review data acquired from the implanted cardiac device and patient. Some interrogators serve a dual purpose of also transmitting program data to the implanted device and are thus referred to as programmer/interrogators. In the program data transmission situation, the physician sets up detection parameters and/or a mode of therapy for the patient. Both program data transmission and cardiac data acquisition are accomplished by RF communication between the implanted device and the programmer/interrogator through the patient's body.
A primary use of the current generation of interrogator apparatus, or alternatively the monitoring mode of programmer/interrogators is as a follow-up tool to evaluate patient data from an implanted device during subsequent visits to the physician's office. This mode is very important for patients who have an implanted defibrillator as there is no other means available for evaluating the condition or effectiveness of the device. Thus, patients have to schedule frequent periodic visits to their physician's office to have the defibrillator device checked, and also have to make a visit whenever they suspect a cardiac anomaly, such as an arrhythmia. Home follow-up devices and methods which rely on EKG data exist, but such devices and methods are often not suitable for defibrillator patients. The only known home follow-up device suitable for defibrillator patients is manufactured by Applicant's assignee.
The next generation of implantable defibrillators are known to have the additional ability to record continuous electrogram, (EGM) data for limited durations whenever detection circuitry determines that a cardiac event such as an arrhythmia is occurring. However, these devices have limited memory available. The limitation on the available memory in the implanted device is primarily due to the limitation on the physical size of memory chips. Designers strive to make the implantable device smaller, thus limiting storage capacity.
Due to the large memory requirement needed for high resolution analog data when digitized for storage, only short intervals of EKG data can be stored. One such device, Ventritex's Cadence Defibrillator, can store sixty seconds of one event, 32 seconds of each of four events, or 16 seconds of each of seven events. The device always discards the last event data in order to store the most recent event, if the data memory is full.
The longer duration of storage of each event or for a series of events, the better the physician is able to reconstruct the etiology of the patient and device interaction. This is important for optimizing future defibrillator program settings. It is also important for the physician to get as much cardiac event history as possible to better diagnose the patient's condition. These needs are in opposition with a limited amount of memory available in the next generation of implanted devices.
This invention provides an interrogation device and method that may be used by the patient at home to retrieve data from the implanted defibrillator and also to store such data for later transfer and review by the physician. The interrogation device can have up to several orders of magnitude more memory than the implanted device. In use, the patient interrogates the implanted device via the interrogator. The interrogator then stores all new data that had not previously been stored. Data may then be reviewed from the interrogator directly, or communicated from the interrogator to a remote receiving apparatus, for example, via a modem or facsimile. The device and method allow the physician access to many more cardiac event data than could be retrieved from storage from the implanted device alone.
The present invention overcomes the deficiencies of the prior art by providing an interrogator for a implanted cardiac defibrillator whose operating data can be interrogated, stored and electronically relayed by the patient to a physician's office at longer intervals or periods.